1. Field of the Invention
The present invention relates to a lead frame and also to a resin-sealed semiconductor device which uses that lead frame.
2. Description of Related Art
FIGS. 6(A), (B), (C) and (D) are, respectively, a plan view of an example of a lead frame of the past, an expanded plan view of an internal lead part thereof, a cross-sectional view of a resin sealed semiconductor devices which uses this lead, and a partial enlarged cross-sectional view about a connecting portion formed between a bonding wire and a lead.
In the past, a lead frame, as shown in FIGS. 6(A) and (B), was formed by a plating the tip of lead 1, which is made from a 42% Ni-Fe alloy or copper, to form a plating layer 2 as a bonding area. This plating layer 2 is formed on the tip of the lead 1 to provide an undercoat that assures electrical conductivity and joining of the wiring bond. Silver plating is usually used as the material for plating layer 2.
Turning to FIG. 6(C), a semiconductor chip is mounted onto a lead frame island 4, the boding wires 8 are made of gold, are connected to terminals formed on the chip, and the bonded portion are sealed with an epoxy resin 7. The connection surfaces formed between the bonding wires 8 and the tip portion of the leads 1, as shown in FIG. 6(D), is a flat configuration formed only by the plating layer 2.
The bonding ball portion which is an end portion of the bonding wire and to be connected to the lead is connected thereto in a crushed shape, the crushed part leaving a shape that is thinner than the wire part.
FIGS. 7(A), (B), and (C) are, respectively, a cross-sectional view of a resin-sealed semiconductor device, which illustrates problems in the prior art lead frame, and cross-sectional views which illustrate the mechanism whereby peeling occurs at a part a thereof.
As shown in FIGS. 7(A), (b), and (C), in a prior art resin-sealed semiconductor device, when the semiconductor is mounted by the user, if the package, has absorbed moisture, by virtue of thermal expansion of moisture within the package, caused by thermal stress, expansion of moisture content occurs at the boundary between the resin 7 and the plating layer 2. This situation leads to the problem of peeling generated at a phase formed between the surface of plating layer 2 and the surface of the sealing resin, from the tip of the lead 1, along the plating layer 2.
The first problem that arises with resin-sealing semiconductor devices in the past is that of broken bonding wires leading to an open connection failure, which occurs because of expansion of water content within the package caused by thermal stress. This happens if the package absorbs water at the time the user mounts the device.
The reason for this is that expansion of the moisture content absorbed therein when it is mounted, usually occurs at the lead tip within the package.
The lead tip plated area is a silver plated area, which oxidizes during the manufacturing processes before resin sealing of the lead frame.
As a result, there is a deterioration of the intimate contact at the boundary between the resin and the plated area, this leading to generate the peeling on the boundary between the resin and the plating layer caused by moisture content expansion, the moisture of which was absorbed when the device is mounted.
This peeling progresses from the lead tip along the plating layer in any direction, as shown by line P in FIG. 7(C), causing an open connection in the bonding wire at the crushed part of the bonding ball, at which the wire is thin.
In view of the above-noted problem with the prior art, an object of the present invention is to provide a highly reliable resin-sealed semiconductor device with improved user mounting characteristics.